In an image display module of electronic devices, such as mobile handheld devices, computer displays and touch panels, a glass or a plastic film is laminated as a surface protective layer. Such a surface protective layer is fixed to an image display module or touch panel by applying a frame-shaped tape or adhesive to the margin outside of the image display portion or outside the active region of the touch panel. As a result, a gap is formed between the image display portion or the active region of the touch panel and the surface protective layer.
There has been a trend in the industry of replacing the gap between the image display module or touch panel and the surface protective layer with a transparent substance that nearly matches the refractive indices of these materials to enhance the transparency and to improve image clarity. Illustrative transparent substances include, but are not limited to: pressure sensitive adhesives, adhesives, silicon gels and the like. When an adhesive is used, it is difficult to separate and replace the surface protective layer when, for example, a defect is produced after laminating the surface protective layer and the image display module. Silicone gel has a reliability problem because of its low adhesive force. On the other hand, a pressure sensitive adhesive (for example, a pressure sensitive adhesive sheet) allows re-lamination, despite a sufficiently high adhesive force, and therefore is effective in laminating a surface protective layer to an image display module or a touch panel.
The surface of an adherend such as an image display module, optical member or surface protective layer is sometimes uneven. The surface of the surface protective layer, particularly the surface coming into contact with the pressure sensitive adhesive sheet, is often subjected to printing for the purpose of decoration or light shielding. In some instances, the printed portion produces a step of 10 μm or more in height on the surface of the surface protective layer. One potential problem with laminating an image display module or a touch panel with a surface protective layer using a pressure sensitive adhesive sheet is that the pressure sensitive adhesive sheet may be insufficient in conformability to the step and causes a gap on or near the step. Furthermore, color unevenness can be generated in a liquid crystal display due to excessively large stress resulting from deformation of the pressure sensitive adhesive. To avoid these problems, the pressure sensitive adhesive sheet thickness should typically be about 10 times the step height. Even when the thickness is as large as 10 times or more of the step height, if a pressure sensitive adhesive with poor stress relaxation is used, laminating requirements may not be satisfied.
Kokai (Japanese Unexamined Patent Publication) No. 2004-262957 describes a hot melt-type ultraviolet cross-linking transparent pressure sensitive adhesive containing, as a radical photoinitiator, at least a hydrogen abstraction-type radical photoinitiator in a ratio of 0.01 to 1.0 wt % based on a (meth)acrylic acid ester-based copolymer, and a pressure sensitive adhesive sheet obtained by hot-melt forming such a pressure sensitive adhesive and irradiating an ultraviolet ray thereon to effect cross-linking.